This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Optical amplifiers are employed in the field of optical transmission technology for amplifying the optical signals transmitted in an optical network. The optical signals in many cases propagate over long links measuring several hundred kilometers and more in an optical fiber, being attenuated in the process. It is therefore necessary to amplify the optical signals when they have been transmitted over a long distance.
Optical links and networks of this type frequently employ Wavelength Division Multiplexing (WDM), a technique whereby a plurality of channels is transmitted in an optical fiber simultaneously at various wavelengths.
Erbium-doped fiber amplifiers (EDFAs) are largely employed in WDM transmission systems. An EDFA operates using an erbium-doped fiber into which the light from an optical pump, for example a laser diode, is coupled. The optical signal launched into the doped fiber is therein amplified by means of stimulated photon emission.
The optical signals are transmitted from one network node (cross-connect, XC) to another network node over a chain of transmission fibers interrupted by inline optical amplifiers.
FIG. 1 is a diagrammatic representation of an optical network 11 with five cross-connects (XC), namely XC1, XC2, XC3, XC4, and XC5. In the cross-connects, signals from many cross-connects are routed to different directions. This can be seen in FIG. 1 as an example: cross-connect XC4 routes the optical signals λ1 and λ2 from the cross-connect XC5 to the cross-connect XC3 and backwards from XC3 to XC5 along the paths 14 and 15; similarly XC4 routes the optical signals λ3 and λ4 from the cross-connect XC5 to the cross-connect XC1 and backwards from the cross-connect XC1 to XC5 along the paths 12 and 15; in a similar way the cross-connect XC4 routes the optical signal λ5 from XC1 to XC3 and backwards from XC3 to XC1 along the paths 12 and 14; in a similar fashion XC4 routes the optical signal λ6 from XC2 to XC3 and backwards from XC3 to XC2 along the paths 13 and 14, and the optical signal λ7 from XC1 to XC2 and backwards from XC2 to XC1 along the paths 12 and 13. The paths 12, 13, 14 and 15 described in FIG. 1 may include two different optical fibers.
The inline amplifier's gain depends on the defined output power Pout, on the properties of the preceding fiber and strongly on the number of channels. As an example, the gain factor G of an inline amplifier 16 located between XC4 and XC5 is G=Pout/Pin, where Pout is the desired sum signal power of the signals λ1, λ2, λ3 and λ4 and Pin is the sum input signal power of the signals λ1, λ2, λ3 and λ4 at the amplifier's input. The sum signal power Pout is defined by the network planner and can be normally a fixed value. The amplifier 16 measures the incoming sum signal power Pin and adapts the gain factor G so that the desired sum output power Pout can be obtained.
FIG. 2 is a diagrammatic representation of an optical network 21, which is similar to the optical network 11 of FIG. 1 but, unlike the optical network 11, it has an interrupted link 28 so that no signal can be transmitted anymore between the cross-connects XC3 and XC4 along the path 24. As a consequence, cross-connect XC4 cannot route the optical signals λ1 and λ2 from the cross-connect XC5 to the cross-connect XC3 and backwards from XC3 to XC5 along the paths 24 and 25, or the optical signal λ5 from XC1 to XC3 and backwards from XC3 to XC1 along the paths 24 and 22, or the optical signal λ6 from XC2 to XC3 and backwards from XC3 to XC2 along the paths 24 and 23. As a further consequence, along the path 25 and in particular on the fiber 251, from the cross-connect XC4 to the cross-connect XC5, only the two backwards signals λ3 and λ4 can be transmitted and not λ1 and λ2. In this way, the amplifier 26 measures the sum power of λ3 and λ4, and this may results in a reduction of the sum input and sum output power of, for example, 3 dB. Although the amplifiers may try to keep the gain constant there could be some overshoots generated by the population of the third energy level and by an imperfect prediction of the required pump power.
The temporary increase of the signal power may result in dynamic impairments, the so-called transients, while static impairments are mainly due to spectral reconfiguration, which may include, for example, non-linear effects, spectral hole burning, intra-band Raman effects and Brillouin scattering.
As a consequence, the planned optical performance cannot be guaranteed anymore as the channel power is increased. Moreover, the signal power at the receiver can be too high, so that high optical power may destroy sensitive components of the receiver, such as, for example, the photo-diode.
A known way to suppress transients is adding additional lasers which replace the signal power of the lost signals, so that the sum input power in an amplifier is kept constant. These lasers are usually continuous wave signals at defined wavelengths. However, this solution may require a high number of additional lasers which reduce the number of wave length channels. Moreover, due to the high optical power of the fill lasers high non-linear effects might occur especially on low dispersion fibre types.
The problem to be solved is to overcome the disadvantages stated above and in particular to provide a solution that in case of a connection interruption suppress transients efficiently without influencing the transmission performance of other channels.